1 #![stable(feature = "rust1", since = "1.0.0")]
3 //! Thread-safe reference-counting pointers.
5 //! See the [`Arc<T>`][Arc] documentation for more details.
9 use core::cmp::Ordering;
10 use core::convert::{From, TryFrom};
12 use core::hash::{Hash, Hasher};
14 use core::intrinsics::abort;
15 #[cfg(not(no_global_oom_handling))]
17 use core::marker::{PhantomData, Unpin, Unsize};
18 #[cfg(not(no_global_oom_handling))]
19 use core::mem::size_of_val;
20 use core::mem::{self, align_of_val_raw};
21 use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
22 use core::panic::{RefUnwindSafe, UnwindSafe};
24 use core::ptr::{self, NonNull};
25 #[cfg(not(no_global_oom_handling))]
26 use core::slice::from_raw_parts_mut;
27 use core::sync::atomic;
28 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
30 #[cfg(not(no_global_oom_handling))]
31 use crate::alloc::handle_alloc_error;
32 #[cfg(not(no_global_oom_handling))]
33 use crate::alloc::{box_free, WriteCloneIntoRaw};
34 use crate::alloc::{AllocError, Allocator, Global, Layout};
35 use crate::borrow::{Cow, ToOwned};
36 use crate::boxed::Box;
37 use crate::rc::is_dangling;
38 #[cfg(not(no_global_oom_handling))]
39 use crate::string::String;
40 #[cfg(not(no_global_oom_handling))]
46 /// A soft limit on the amount of references that may be made to an `Arc`.
48 /// Going above this limit will abort your program (although not
49 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
50 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
52 #[cfg(not(sanitize = "thread"))]
53 macro_rules! acquire {
55 atomic::fence(Acquire)
59 // ThreadSanitizer does not support memory fences. To avoid false positive
60 // reports in Arc / Weak implementation use atomic loads for synchronization
62 #[cfg(sanitize = "thread")]
63 macro_rules! acquire {
69 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
70 /// Reference Counted'.
72 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
73 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
74 /// a new `Arc` instance, which points to the same allocation on the heap as the
75 /// source `Arc`, while increasing a reference count. When the last `Arc`
76 /// pointer to a given allocation is destroyed, the value stored in that allocation (often
77 /// referred to as "inner value") is also dropped.
79 /// Shared references in Rust disallow mutation by default, and `Arc` is no
80 /// exception: you cannot generally obtain a mutable reference to something
81 /// inside an `Arc`. If you need to mutate through an `Arc`, use
82 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
87 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
88 /// counting. This means that it is thread-safe. The disadvantage is that
89 /// atomic operations are more expensive than ordinary memory accesses. If you
90 /// are not sharing reference-counted allocations between threads, consider using
91 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
92 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
93 /// However, a library might choose `Arc<T>` in order to give library consumers
96 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
97 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
98 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
99 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
100 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
101 /// data, but it doesn't add thread safety to its data. Consider
102 /// <code>Arc<[RefCell\<T>]></code>. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
103 /// [`Send`], <code>Arc<[RefCell\<T>]></code> would be as well. But then we'd have a problem:
104 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
105 /// non-atomic operations.
107 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
108 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
110 /// ## Breaking cycles with `Weak`
112 /// The [`downgrade`][downgrade] method can be used to create a non-owning
113 /// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
114 /// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
115 /// already been dropped. In other words, `Weak` pointers do not keep the value
116 /// inside the allocation alive; however, they *do* keep the allocation
117 /// (the backing store for the value) alive.
119 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
120 /// [`Weak`] is used to break cycles. For example, a tree could have
121 /// strong `Arc` pointers from parent nodes to children, and [`Weak`]
122 /// pointers from children back to their parents.
124 /// # Cloning references
126 /// Creating a new reference from an existing reference-counted pointer is done using the
127 /// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
130 /// use std::sync::Arc;
131 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
132 /// // The two syntaxes below are equivalent.
133 /// let a = foo.clone();
134 /// let b = Arc::clone(&foo);
135 /// // a, b, and foo are all Arcs that point to the same memory location
138 /// ## `Deref` behavior
140 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
141 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
142 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
143 /// functions, called using [fully qualified syntax]:
146 /// use std::sync::Arc;
148 /// let my_arc = Arc::new(());
149 /// Arc::downgrade(&my_arc);
152 /// `Arc<T>`'s implementations of traits like `Clone` may also be called using
153 /// fully qualified syntax. Some people prefer to use fully qualified syntax,
154 /// while others prefer using method-call syntax.
157 /// use std::sync::Arc;
159 /// let arc = Arc::new(());
160 /// // Method-call syntax
161 /// let arc2 = arc.clone();
162 /// // Fully qualified syntax
163 /// let arc3 = Arc::clone(&arc);
166 /// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
167 /// already been dropped.
169 /// [`Rc<T>`]: crate::rc::Rc
170 /// [clone]: Clone::clone
171 /// [mutex]: ../../std/sync/struct.Mutex.html
172 /// [rwlock]: ../../std/sync/struct.RwLock.html
173 /// [atomic]: core::sync::atomic
174 /// [`Send`]: core::marker::Send
175 /// [`Sync`]: core::marker::Sync
176 /// [deref]: core::ops::Deref
177 /// [downgrade]: Arc::downgrade
178 /// [upgrade]: Weak::upgrade
179 /// [RefCell\<T>]: core::cell::RefCell
180 /// [`RefCell<T>`]: core::cell::RefCell
181 /// [`std::sync`]: ../../std/sync/index.html
182 /// [`Arc::clone(&from)`]: Arc::clone
183 /// [fully qualified syntax]: https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#fully-qualified-syntax-for-disambiguation-calling-methods-with-the-same-name
187 /// Sharing some immutable data between threads:
189 // Note that we **do not** run these tests here. The windows builders get super
190 // unhappy if a thread outlives the main thread and then exits at the same time
191 // (something deadlocks) so we just avoid this entirely by not running these
194 /// use std::sync::Arc;
197 /// let five = Arc::new(5);
200 /// let five = Arc::clone(&five);
202 /// thread::spawn(move || {
203 /// println!("{:?}", five);
208 /// Sharing a mutable [`AtomicUsize`]:
210 /// [`AtomicUsize`]: core::sync::atomic::AtomicUsize "sync::atomic::AtomicUsize"
213 /// use std::sync::Arc;
214 /// use std::sync::atomic::{AtomicUsize, Ordering};
217 /// let val = Arc::new(AtomicUsize::new(5));
220 /// let val = Arc::clone(&val);
222 /// thread::spawn(move || {
223 /// let v = val.fetch_add(1, Ordering::SeqCst);
224 /// println!("{:?}", v);
229 /// See the [`rc` documentation][rc_examples] for more examples of reference
230 /// counting in general.
232 /// [rc_examples]: crate::rc#examples
233 #[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
234 #[stable(feature = "rust1", since = "1.0.0")]
235 pub struct Arc<T: ?Sized> {
236 ptr: NonNull<ArcInner<T>>,
237 phantom: PhantomData<ArcInner<T>>,
240 #[stable(feature = "rust1", since = "1.0.0")]
241 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
242 #[stable(feature = "rust1", since = "1.0.0")]
243 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
245 #[stable(feature = "catch_unwind", since = "1.9.0")]
246 impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Arc<T> {}
248 #[unstable(feature = "coerce_unsized", issue = "27732")]
249 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
251 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
252 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
254 impl<T: ?Sized> Arc<T> {
255 fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
256 Self { ptr, phantom: PhantomData }
259 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
260 unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
264 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
265 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
266 /// pointer, which returns an <code>[Option]<[Arc]\<T>></code>.
268 /// Since a `Weak` reference does not count towards ownership, it will not
269 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
270 /// guarantees about the value still being present. Thus it may return [`None`]
271 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
272 /// itself (the backing store) from being deallocated.
274 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
275 /// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
276 /// prevent circular references between [`Arc`] pointers, since mutual owning references
277 /// would never allow either [`Arc`] to be dropped. For example, a tree could
278 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
279 /// pointers from children back to their parents.
281 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
283 /// [`upgrade`]: Weak::upgrade
284 #[stable(feature = "arc_weak", since = "1.4.0")]
285 pub struct Weak<T: ?Sized> {
286 // This is a `NonNull` to allow optimizing the size of this type in enums,
287 // but it is not necessarily a valid pointer.
288 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
289 // to allocate space on the heap. That's not a value a real pointer
290 // will ever have because RcBox has alignment at least 2.
291 // This is only possible when `T: Sized`; unsized `T` never dangle.
292 ptr: NonNull<ArcInner<T>>,
295 #[stable(feature = "arc_weak", since = "1.4.0")]
296 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
297 #[stable(feature = "arc_weak", since = "1.4.0")]
298 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
300 #[unstable(feature = "coerce_unsized", issue = "27732")]
301 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
302 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
303 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
305 #[stable(feature = "arc_weak", since = "1.4.0")]
306 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
307 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
312 // This is repr(C) to future-proof against possible field-reordering, which
313 // would interfere with otherwise safe [into|from]_raw() of transmutable
316 struct ArcInner<T: ?Sized> {
317 strong: atomic::AtomicUsize,
319 // the value usize::MAX acts as a sentinel for temporarily "locking" the
320 // ability to upgrade weak pointers or downgrade strong ones; this is used
321 // to avoid races in `make_mut` and `get_mut`.
322 weak: atomic::AtomicUsize,
327 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
328 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
331 /// Constructs a new `Arc<T>`.
336 /// use std::sync::Arc;
338 /// let five = Arc::new(5);
340 #[cfg(not(no_global_oom_handling))]
342 #[stable(feature = "rust1", since = "1.0.0")]
343 pub fn new(data: T) -> Arc<T> {
344 // Start the weak pointer count as 1 which is the weak pointer that's
345 // held by all the strong pointers (kinda), see std/rc.rs for more info
346 let x: Box<_> = box ArcInner {
347 strong: atomic::AtomicUsize::new(1),
348 weak: atomic::AtomicUsize::new(1),
351 Self::from_inner(Box::leak(x).into())
354 /// Constructs a new `Arc<T>` using a weak reference to itself. Attempting
355 /// to upgrade the weak reference before this function returns will result
356 /// in a `None` value. However, the weak reference may be cloned freely and
357 /// stored for use at a later time.
361 /// #![feature(arc_new_cyclic)]
362 /// #![allow(dead_code)]
364 /// use std::sync::{Arc, Weak};
370 /// let foo = Arc::new_cyclic(|me| Foo {
374 #[cfg(not(no_global_oom_handling))]
376 #[unstable(feature = "arc_new_cyclic", issue = "75861")]
377 pub fn new_cyclic(data_fn: impl FnOnce(&Weak<T>) -> T) -> Arc<T> {
378 // Construct the inner in the "uninitialized" state with a single
380 let uninit_ptr: NonNull<_> = Box::leak(box ArcInner {
381 strong: atomic::AtomicUsize::new(0),
382 weak: atomic::AtomicUsize::new(1),
383 data: mem::MaybeUninit::<T>::uninit(),
386 let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
388 let weak = Weak { ptr: init_ptr };
390 // It's important we don't give up ownership of the weak pointer, or
391 // else the memory might be freed by the time `data_fn` returns. If
392 // we really wanted to pass ownership, we could create an additional
393 // weak pointer for ourselves, but this would result in additional
394 // updates to the weak reference count which might not be necessary
396 let data = data_fn(&weak);
398 // Now we can properly initialize the inner value and turn our weak
399 // reference into a strong reference.
401 let inner = init_ptr.as_ptr();
402 ptr::write(ptr::addr_of_mut!((*inner).data), data);
404 // The above write to the data field must be visible to any threads which
405 // observe a non-zero strong count. Therefore we need at least "Release" ordering
406 // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
408 // "Acquire" ordering is not required. When considering the possible behaviours
409 // of `data_fn` we only need to look at what it could do with a reference to a
410 // non-upgradeable `Weak`:
411 // - It can *clone* the `Weak`, increasing the weak reference count.
412 // - It can drop those clones, decreasing the weak reference count (but never to zero).
414 // These side effects do not impact us in any way, and no other side effects are
415 // possible with safe code alone.
416 let prev_value = (*inner).strong.fetch_add(1, Release);
417 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
420 let strong = Arc::from_inner(init_ptr);
422 // Strong references should collectively own a shared weak reference,
423 // so don't run the destructor for our old weak reference.
428 /// Constructs a new `Arc` with uninitialized contents.
433 /// #![feature(new_uninit)]
434 /// #![feature(get_mut_unchecked)]
436 /// use std::sync::Arc;
438 /// let mut five = Arc::<u32>::new_uninit();
440 /// let five = unsafe {
441 /// // Deferred initialization:
442 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
444 /// five.assume_init()
447 /// assert_eq!(*five, 5)
449 #[cfg(not(no_global_oom_handling))]
450 #[unstable(feature = "new_uninit", issue = "63291")]
452 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
454 Arc::from_ptr(Arc::allocate_for_layout(
456 |layout| Global.allocate(layout),
457 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
462 /// Constructs a new `Arc` with uninitialized contents, with the memory
463 /// being filled with `0` bytes.
465 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
471 /// #![feature(new_uninit)]
473 /// use std::sync::Arc;
475 /// let zero = Arc::<u32>::new_zeroed();
476 /// let zero = unsafe { zero.assume_init() };
478 /// assert_eq!(*zero, 0)
481 /// [zeroed]: mem::MaybeUninit::zeroed
482 #[cfg(not(no_global_oom_handling))]
483 #[unstable(feature = "new_uninit", issue = "63291")]
485 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
487 Arc::from_ptr(Arc::allocate_for_layout(
489 |layout| Global.allocate_zeroed(layout),
490 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
495 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
496 /// `data` will be pinned in memory and unable to be moved.
497 #[cfg(not(no_global_oom_handling))]
498 #[stable(feature = "pin", since = "1.33.0")]
500 pub fn pin(data: T) -> Pin<Arc<T>> {
501 unsafe { Pin::new_unchecked(Arc::new(data)) }
504 /// Constructs a new `Pin<Arc<T>>`, return an error if allocation fails.
505 #[unstable(feature = "allocator_api", issue = "32838")]
507 pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError> {
508 unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
511 /// Constructs a new `Arc<T>`, returning an error if allocation fails.
516 /// #![feature(allocator_api)]
517 /// use std::sync::Arc;
519 /// let five = Arc::try_new(5)?;
520 /// # Ok::<(), std::alloc::AllocError>(())
522 #[unstable(feature = "allocator_api", issue = "32838")]
524 pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
525 // Start the weak pointer count as 1 which is the weak pointer that's
526 // held by all the strong pointers (kinda), see std/rc.rs for more info
527 let x: Box<_> = Box::try_new(ArcInner {
528 strong: atomic::AtomicUsize::new(1),
529 weak: atomic::AtomicUsize::new(1),
532 Ok(Self::from_inner(Box::leak(x).into()))
535 /// Constructs a new `Arc` with uninitialized contents, returning an error
536 /// if allocation fails.
541 /// #![feature(new_uninit, allocator_api)]
542 /// #![feature(get_mut_unchecked)]
544 /// use std::sync::Arc;
546 /// let mut five = Arc::<u32>::try_new_uninit()?;
548 /// let five = unsafe {
549 /// // Deferred initialization:
550 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
552 /// five.assume_init()
555 /// assert_eq!(*five, 5);
556 /// # Ok::<(), std::alloc::AllocError>(())
558 #[unstable(feature = "allocator_api", issue = "32838")]
559 // #[unstable(feature = "new_uninit", issue = "63291")]
560 pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
562 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
564 |layout| Global.allocate(layout),
565 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
570 /// Constructs a new `Arc` with uninitialized contents, with the memory
571 /// being filled with `0` bytes, returning an error if allocation fails.
573 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
579 /// #![feature(new_uninit, allocator_api)]
581 /// use std::sync::Arc;
583 /// let zero = Arc::<u32>::try_new_zeroed()?;
584 /// let zero = unsafe { zero.assume_init() };
586 /// assert_eq!(*zero, 0);
587 /// # Ok::<(), std::alloc::AllocError>(())
590 /// [zeroed]: mem::MaybeUninit::zeroed
591 #[unstable(feature = "allocator_api", issue = "32838")]
592 // #[unstable(feature = "new_uninit", issue = "63291")]
593 pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
595 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
597 |layout| Global.allocate_zeroed(layout),
598 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
602 /// Returns the inner value, if the `Arc` has exactly one strong reference.
604 /// Otherwise, an [`Err`] is returned with the same `Arc` that was
607 /// This will succeed even if there are outstanding weak references.
612 /// use std::sync::Arc;
614 /// let x = Arc::new(3);
615 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
617 /// let x = Arc::new(4);
618 /// let _y = Arc::clone(&x);
619 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
622 #[stable(feature = "arc_unique", since = "1.4.0")]
623 pub fn try_unwrap(this: Self) -> Result<T, Self> {
624 if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
628 acquire!(this.inner().strong);
631 let elem = ptr::read(&this.ptr.as_ref().data);
633 // Make a weak pointer to clean up the implicit strong-weak reference
634 let _weak = Weak { ptr: this.ptr };
643 /// Constructs a new atomically reference-counted slice with uninitialized contents.
648 /// #![feature(new_uninit)]
649 /// #![feature(get_mut_unchecked)]
651 /// use std::sync::Arc;
653 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
655 /// let values = unsafe {
656 /// // Deferred initialization:
657 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
658 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
659 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
661 /// values.assume_init()
664 /// assert_eq!(*values, [1, 2, 3])
666 #[cfg(not(no_global_oom_handling))]
667 #[unstable(feature = "new_uninit", issue = "63291")]
669 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
670 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
673 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
674 /// filled with `0` bytes.
676 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
677 /// incorrect usage of this method.
682 /// #![feature(new_uninit)]
684 /// use std::sync::Arc;
686 /// let values = Arc::<[u32]>::new_zeroed_slice(3);
687 /// let values = unsafe { values.assume_init() };
689 /// assert_eq!(*values, [0, 0, 0])
692 /// [zeroed]: mem::MaybeUninit::zeroed
693 #[cfg(not(no_global_oom_handling))]
694 #[unstable(feature = "new_uninit", issue = "63291")]
696 pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
698 Arc::from_ptr(Arc::allocate_for_layout(
699 Layout::array::<T>(len).unwrap(),
700 |layout| Global.allocate_zeroed(layout),
702 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
703 as *mut ArcInner<[mem::MaybeUninit<T>]>
710 impl<T> Arc<mem::MaybeUninit<T>> {
711 /// Converts to `Arc<T>`.
715 /// As with [`MaybeUninit::assume_init`],
716 /// it is up to the caller to guarantee that the inner value
717 /// really is in an initialized state.
718 /// Calling this when the content is not yet fully initialized
719 /// causes immediate undefined behavior.
721 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
726 /// #![feature(new_uninit)]
727 /// #![feature(get_mut_unchecked)]
729 /// use std::sync::Arc;
731 /// let mut five = Arc::<u32>::new_uninit();
733 /// let five = unsafe {
734 /// // Deferred initialization:
735 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
737 /// five.assume_init()
740 /// assert_eq!(*five, 5)
742 #[unstable(feature = "new_uninit", issue = "63291")]
743 #[must_use = "`self` will be dropped if the result is not used"]
745 pub unsafe fn assume_init(self) -> Arc<T> {
746 Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
750 impl<T> Arc<[mem::MaybeUninit<T>]> {
751 /// Converts to `Arc<[T]>`.
755 /// As with [`MaybeUninit::assume_init`],
756 /// it is up to the caller to guarantee that the inner value
757 /// really is in an initialized state.
758 /// Calling this when the content is not yet fully initialized
759 /// causes immediate undefined behavior.
761 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
766 /// #![feature(new_uninit)]
767 /// #![feature(get_mut_unchecked)]
769 /// use std::sync::Arc;
771 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
773 /// let values = unsafe {
774 /// // Deferred initialization:
775 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
776 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
777 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
779 /// values.assume_init()
782 /// assert_eq!(*values, [1, 2, 3])
784 #[unstable(feature = "new_uninit", issue = "63291")]
785 #[must_use = "`self` will be dropped if the result is not used"]
787 pub unsafe fn assume_init(self) -> Arc<[T]> {
788 unsafe { Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
792 impl<T: ?Sized> Arc<T> {
793 /// Consumes the `Arc`, returning the wrapped pointer.
795 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
796 /// [`Arc::from_raw`].
801 /// use std::sync::Arc;
803 /// let x = Arc::new("hello".to_owned());
804 /// let x_ptr = Arc::into_raw(x);
805 /// assert_eq!(unsafe { &*x_ptr }, "hello");
807 #[stable(feature = "rc_raw", since = "1.17.0")]
808 pub fn into_raw(this: Self) -> *const T {
809 let ptr = Self::as_ptr(&this);
814 /// Provides a raw pointer to the data.
816 /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
817 /// as long as there are strong counts in the `Arc`.
822 /// use std::sync::Arc;
824 /// let x = Arc::new("hello".to_owned());
825 /// let y = Arc::clone(&x);
826 /// let x_ptr = Arc::as_ptr(&x);
827 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
828 /// assert_eq!(unsafe { &*x_ptr }, "hello");
830 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
831 pub fn as_ptr(this: &Self) -> *const T {
832 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
834 // SAFETY: This cannot go through Deref::deref or RcBoxPtr::inner because
835 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
836 // write through the pointer after the Rc is recovered through `from_raw`.
837 unsafe { ptr::addr_of_mut!((*ptr).data) }
840 /// Constructs an `Arc<T>` from a raw pointer.
842 /// The raw pointer must have been previously returned by a call to
843 /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
844 /// alignment as `T`. This is trivially true if `U` is `T`.
845 /// Note that if `U` is not `T` but has the same size and alignment, this is
846 /// basically like transmuting references of different types. See
847 /// [`mem::transmute`][transmute] for more information on what
848 /// restrictions apply in this case.
850 /// The user of `from_raw` has to make sure a specific value of `T` is only
853 /// This function is unsafe because improper use may lead to memory unsafety,
854 /// even if the returned `Arc<T>` is never accessed.
856 /// [into_raw]: Arc::into_raw
857 /// [transmute]: core::mem::transmute
862 /// use std::sync::Arc;
864 /// let x = Arc::new("hello".to_owned());
865 /// let x_ptr = Arc::into_raw(x);
868 /// // Convert back to an `Arc` to prevent leak.
869 /// let x = Arc::from_raw(x_ptr);
870 /// assert_eq!(&*x, "hello");
872 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
875 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
877 #[stable(feature = "rc_raw", since = "1.17.0")]
878 pub unsafe fn from_raw(ptr: *const T) -> Self {
880 let offset = data_offset(ptr);
882 // Reverse the offset to find the original ArcInner.
883 let arc_ptr = (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset));
885 Self::from_ptr(arc_ptr)
889 /// Creates a new [`Weak`] pointer to this allocation.
894 /// use std::sync::Arc;
896 /// let five = Arc::new(5);
898 /// let weak_five = Arc::downgrade(&five);
900 #[stable(feature = "arc_weak", since = "1.4.0")]
901 pub fn downgrade(this: &Self) -> Weak<T> {
902 // This Relaxed is OK because we're checking the value in the CAS
904 let mut cur = this.inner().weak.load(Relaxed);
907 // check if the weak counter is currently "locked"; if so, spin.
908 if cur == usize::MAX {
910 cur = this.inner().weak.load(Relaxed);
914 // NOTE: this code currently ignores the possibility of overflow
915 // into usize::MAX; in general both Rc and Arc need to be adjusted
916 // to deal with overflow.
918 // Unlike with Clone(), we need this to be an Acquire read to
919 // synchronize with the write coming from `is_unique`, so that the
920 // events prior to that write happen before this read.
921 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
923 // Make sure we do not create a dangling Weak
924 debug_assert!(!is_dangling(this.ptr.as_ptr()));
925 return Weak { ptr: this.ptr };
927 Err(old) => cur = old,
932 /// Gets the number of [`Weak`] pointers to this allocation.
936 /// This method by itself is safe, but using it correctly requires extra care.
937 /// Another thread can change the weak count at any time,
938 /// including potentially between calling this method and acting on the result.
943 /// use std::sync::Arc;
945 /// let five = Arc::new(5);
946 /// let _weak_five = Arc::downgrade(&five);
948 /// // This assertion is deterministic because we haven't shared
949 /// // the `Arc` or `Weak` between threads.
950 /// assert_eq!(1, Arc::weak_count(&five));
953 #[stable(feature = "arc_counts", since = "1.15.0")]
954 pub fn weak_count(this: &Self) -> usize {
955 let cnt = this.inner().weak.load(SeqCst);
956 // If the weak count is currently locked, the value of the
957 // count was 0 just before taking the lock.
958 if cnt == usize::MAX { 0 } else { cnt - 1 }
961 /// Gets the number of strong (`Arc`) pointers to this allocation.
965 /// This method by itself is safe, but using it correctly requires extra care.
966 /// Another thread can change the strong count at any time,
967 /// including potentially between calling this method and acting on the result.
972 /// use std::sync::Arc;
974 /// let five = Arc::new(5);
975 /// let _also_five = Arc::clone(&five);
977 /// // This assertion is deterministic because we haven't shared
978 /// // the `Arc` between threads.
979 /// assert_eq!(2, Arc::strong_count(&five));
982 #[stable(feature = "arc_counts", since = "1.15.0")]
983 pub fn strong_count(this: &Self) -> usize {
984 this.inner().strong.load(SeqCst)
987 /// Increments the strong reference count on the `Arc<T>` associated with the
988 /// provided pointer by one.
992 /// The pointer must have been obtained through `Arc::into_raw`, and the
993 /// associated `Arc` instance must be valid (i.e. the strong count must be at
994 /// least 1) for the duration of this method.
999 /// use std::sync::Arc;
1001 /// let five = Arc::new(5);
1004 /// let ptr = Arc::into_raw(five);
1005 /// Arc::increment_strong_count(ptr);
1007 /// // This assertion is deterministic because we haven't shared
1008 /// // the `Arc` between threads.
1009 /// let five = Arc::from_raw(ptr);
1010 /// assert_eq!(2, Arc::strong_count(&five));
1014 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1015 pub unsafe fn increment_strong_count(ptr: *const T) {
1016 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
1017 let arc = unsafe { mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr)) };
1018 // Now increase refcount, but don't drop new refcount either
1019 let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
1022 /// Decrements the strong reference count on the `Arc<T>` associated with the
1023 /// provided pointer by one.
1027 /// The pointer must have been obtained through `Arc::into_raw`, and the
1028 /// associated `Arc` instance must be valid (i.e. the strong count must be at
1029 /// least 1) when invoking this method. This method can be used to release the final
1030 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1036 /// use std::sync::Arc;
1038 /// let five = Arc::new(5);
1041 /// let ptr = Arc::into_raw(five);
1042 /// Arc::increment_strong_count(ptr);
1044 /// // Those assertions are deterministic because we haven't shared
1045 /// // the `Arc` between threads.
1046 /// let five = Arc::from_raw(ptr);
1047 /// assert_eq!(2, Arc::strong_count(&five));
1048 /// Arc::decrement_strong_count(ptr);
1049 /// assert_eq!(1, Arc::strong_count(&five));
1053 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1054 pub unsafe fn decrement_strong_count(ptr: *const T) {
1055 unsafe { mem::drop(Arc::from_raw(ptr)) };
1059 fn inner(&self) -> &ArcInner<T> {
1060 // This unsafety is ok because while this arc is alive we're guaranteed
1061 // that the inner pointer is valid. Furthermore, we know that the
1062 // `ArcInner` structure itself is `Sync` because the inner data is
1063 // `Sync` as well, so we're ok loaning out an immutable pointer to these
1065 unsafe { self.ptr.as_ref() }
1068 // Non-inlined part of `drop`.
1070 unsafe fn drop_slow(&mut self) {
1071 // Destroy the data at this time, even though we must not free the box
1072 // allocation itself (there might still be weak pointers lying around).
1073 unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
1075 // Drop the weak ref collectively held by all strong references
1076 drop(Weak { ptr: self.ptr });
1080 #[stable(feature = "ptr_eq", since = "1.17.0")]
1081 /// Returns `true` if the two `Arc`s point to the same allocation
1082 /// (in a vein similar to [`ptr::eq`]).
1087 /// use std::sync::Arc;
1089 /// let five = Arc::new(5);
1090 /// let same_five = Arc::clone(&five);
1091 /// let other_five = Arc::new(5);
1093 /// assert!(Arc::ptr_eq(&five, &same_five));
1094 /// assert!(!Arc::ptr_eq(&five, &other_five));
1097 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
1098 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1099 this.ptr.as_ptr() == other.ptr.as_ptr()
1103 impl<T: ?Sized> Arc<T> {
1104 /// Allocates an `ArcInner<T>` with sufficient space for
1105 /// a possibly-unsized inner value where the value has the layout provided.
1107 /// The function `mem_to_arcinner` is called with the data pointer
1108 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1109 #[cfg(not(no_global_oom_handling))]
1110 unsafe fn allocate_for_layout(
1111 value_layout: Layout,
1112 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1113 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1114 ) -> *mut ArcInner<T> {
1115 // Calculate layout using the given value layout.
1116 // Previously, layout was calculated on the expression
1117 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1118 // reference (see #54908).
1119 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1121 Arc::try_allocate_for_layout(value_layout, allocate, mem_to_arcinner)
1122 .unwrap_or_else(|_| handle_alloc_error(layout))
1126 /// Allocates an `ArcInner<T>` with sufficient space for
1127 /// a possibly-unsized inner value where the value has the layout provided,
1128 /// returning an error if allocation fails.
1130 /// The function `mem_to_arcinner` is called with the data pointer
1131 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1132 unsafe fn try_allocate_for_layout(
1133 value_layout: Layout,
1134 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1135 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1136 ) -> Result<*mut ArcInner<T>, AllocError> {
1137 // Calculate layout using the given value layout.
1138 // Previously, layout was calculated on the expression
1139 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1140 // reference (see #54908).
1141 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1143 let ptr = allocate(layout)?;
1145 // Initialize the ArcInner
1146 let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
1147 debug_assert_eq!(unsafe { Layout::for_value(&*inner) }, layout);
1150 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
1151 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
1157 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
1158 #[cfg(not(no_global_oom_handling))]
1159 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
1160 // Allocate for the `ArcInner<T>` using the given value.
1162 Self::allocate_for_layout(
1163 Layout::for_value(&*ptr),
1164 |layout| Global.allocate(layout),
1165 |mem| (ptr as *mut ArcInner<T>).set_ptr_value(mem) as *mut ArcInner<T>,
1170 #[cfg(not(no_global_oom_handling))]
1171 fn from_box(v: Box<T>) -> Arc<T> {
1173 let (box_unique, alloc) = Box::into_unique(v);
1174 let bptr = box_unique.as_ptr();
1176 let value_size = size_of_val(&*bptr);
1177 let ptr = Self::allocate_for_ptr(bptr);
1179 // Copy value as bytes
1180 ptr::copy_nonoverlapping(
1181 bptr as *const T as *const u8,
1182 &mut (*ptr).data as *mut _ as *mut u8,
1186 // Free the allocation without dropping its contents
1187 box_free(box_unique, alloc);
1195 /// Allocates an `ArcInner<[T]>` with the given length.
1196 #[cfg(not(no_global_oom_handling))]
1197 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
1199 Self::allocate_for_layout(
1200 Layout::array::<T>(len).unwrap(),
1201 |layout| Global.allocate(layout),
1202 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
1207 /// Copy elements from slice into newly allocated Arc<\[T\]>
1209 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
1210 #[cfg(not(no_global_oom_handling))]
1211 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
1213 let ptr = Self::allocate_for_slice(v.len());
1215 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
1221 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
1223 /// Behavior is undefined should the size be wrong.
1224 #[cfg(not(no_global_oom_handling))]
1225 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
1226 // Panic guard while cloning T elements.
1227 // In the event of a panic, elements that have been written
1228 // into the new ArcInner will be dropped, then the memory freed.
1236 impl<T> Drop for Guard<T> {
1237 fn drop(&mut self) {
1239 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1240 ptr::drop_in_place(slice);
1242 Global.deallocate(self.mem, self.layout);
1248 let ptr = Self::allocate_for_slice(len);
1250 let mem = ptr as *mut _ as *mut u8;
1251 let layout = Layout::for_value(&*ptr);
1253 // Pointer to first element
1254 let elems = &mut (*ptr).data as *mut [T] as *mut T;
1256 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1258 for (i, item) in iter.enumerate() {
1259 ptr::write(elems.add(i), item);
1263 // All clear. Forget the guard so it doesn't free the new ArcInner.
1271 /// Specialization trait used for `From<&[T]>`.
1272 #[cfg(not(no_global_oom_handling))]
1273 trait ArcFromSlice<T> {
1274 fn from_slice(slice: &[T]) -> Self;
1277 #[cfg(not(no_global_oom_handling))]
1278 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
1280 default fn from_slice(v: &[T]) -> Self {
1281 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1285 #[cfg(not(no_global_oom_handling))]
1286 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
1288 fn from_slice(v: &[T]) -> Self {
1289 unsafe { Arc::copy_from_slice(v) }
1293 #[stable(feature = "rust1", since = "1.0.0")]
1294 impl<T: ?Sized> Clone for Arc<T> {
1295 /// Makes a clone of the `Arc` pointer.
1297 /// This creates another pointer to the same allocation, increasing the
1298 /// strong reference count.
1303 /// use std::sync::Arc;
1305 /// let five = Arc::new(5);
1307 /// let _ = Arc::clone(&five);
1310 fn clone(&self) -> Arc<T> {
1311 // Using a relaxed ordering is alright here, as knowledge of the
1312 // original reference prevents other threads from erroneously deleting
1315 // As explained in the [Boost documentation][1], Increasing the
1316 // reference counter can always be done with memory_order_relaxed: New
1317 // references to an object can only be formed from an existing
1318 // reference, and passing an existing reference from one thread to
1319 // another must already provide any required synchronization.
1321 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1322 let old_size = self.inner().strong.fetch_add(1, Relaxed);
1324 // However we need to guard against massive refcounts in case someone
1325 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
1326 // and users will use-after free. We racily saturate to `isize::MAX` on
1327 // the assumption that there aren't ~2 billion threads incrementing
1328 // the reference count at once. This branch will never be taken in
1329 // any realistic program.
1331 // We abort because such a program is incredibly degenerate, and we
1332 // don't care to support it.
1333 if old_size > MAX_REFCOUNT {
1337 Self::from_inner(self.ptr)
1341 #[stable(feature = "rust1", since = "1.0.0")]
1342 impl<T: ?Sized> Deref for Arc<T> {
1346 fn deref(&self) -> &T {
1351 #[unstable(feature = "receiver_trait", issue = "none")]
1352 impl<T: ?Sized> Receiver for Arc<T> {}
1354 impl<T: Clone> Arc<T> {
1355 /// Makes a mutable reference into the given `Arc`.
1357 /// If there are other `Arc` pointers to the same allocation, then `make_mut` will
1358 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
1359 /// referred to as clone-on-write.
1361 /// However, if there are no other `Arc` pointers to this allocation, but some [`Weak`]
1362 /// pointers, then the [`Weak`] pointers will be disassociated and the inner value will not
1365 /// See also [`get_mut`], which will fail rather than cloning the inner value
1366 /// or diassociating [`Weak`] pointers.
1368 /// [`clone`]: Clone::clone
1369 /// [`get_mut`]: Arc::get_mut
1374 /// use std::sync::Arc;
1376 /// let mut data = Arc::new(5);
1378 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1379 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1380 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1381 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1382 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1384 /// // Now `data` and `other_data` point to different allocations.
1385 /// assert_eq!(*data, 8);
1386 /// assert_eq!(*other_data, 12);
1389 /// [`Weak`] pointers will be disassociated:
1392 /// use std::sync::Arc;
1394 /// let mut data = Arc::new(75);
1395 /// let weak = Arc::downgrade(&data);
1397 /// assert!(75 == *data);
1398 /// assert!(75 == *weak.upgrade().unwrap());
1400 /// *Arc::make_mut(&mut data) += 1;
1402 /// assert!(76 == *data);
1403 /// assert!(weak.upgrade().is_none());
1405 #[cfg(not(no_global_oom_handling))]
1407 #[stable(feature = "arc_unique", since = "1.4.0")]
1408 pub fn make_mut(this: &mut Self) -> &mut T {
1409 // Note that we hold both a strong reference and a weak reference.
1410 // Thus, releasing our strong reference only will not, by itself, cause
1411 // the memory to be deallocated.
1413 // Use Acquire to ensure that we see any writes to `weak` that happen
1414 // before release writes (i.e., decrements) to `strong`. Since we hold a
1415 // weak count, there's no chance the ArcInner itself could be
1417 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1418 // Another strong pointer exists, so we must clone.
1419 // Pre-allocate memory to allow writing the cloned value directly.
1420 let mut arc = Self::new_uninit();
1422 let data = Arc::get_mut_unchecked(&mut arc);
1423 (**this).write_clone_into_raw(data.as_mut_ptr());
1424 *this = arc.assume_init();
1426 } else if this.inner().weak.load(Relaxed) != 1 {
1427 // Relaxed suffices in the above because this is fundamentally an
1428 // optimization: we are always racing with weak pointers being
1429 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1431 // We removed the last strong ref, but there are additional weak
1432 // refs remaining. We'll move the contents to a new Arc, and
1433 // invalidate the other weak refs.
1435 // Note that it is not possible for the read of `weak` to yield
1436 // usize::MAX (i.e., locked), since the weak count can only be
1437 // locked by a thread with a strong reference.
1439 // Materialize our own implicit weak pointer, so that it can clean
1440 // up the ArcInner as needed.
1441 let _weak = Weak { ptr: this.ptr };
1443 // Can just steal the data, all that's left is Weaks
1444 let mut arc = Self::new_uninit();
1446 let data = Arc::get_mut_unchecked(&mut arc);
1447 data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
1448 ptr::write(this, arc.assume_init());
1451 // We were the sole reference of either kind; bump back up the
1452 // strong ref count.
1453 this.inner().strong.store(1, Release);
1456 // As with `get_mut()`, the unsafety is ok because our reference was
1457 // either unique to begin with, or became one upon cloning the contents.
1458 unsafe { Self::get_mut_unchecked(this) }
1462 impl<T: ?Sized> Arc<T> {
1463 /// Returns a mutable reference into the given `Arc`, if there are
1464 /// no other `Arc` or [`Weak`] pointers to the same allocation.
1466 /// Returns [`None`] otherwise, because it is not safe to
1467 /// mutate a shared value.
1469 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1470 /// the inner value when there are other `Arc` pointers.
1472 /// [make_mut]: Arc::make_mut
1473 /// [clone]: Clone::clone
1478 /// use std::sync::Arc;
1480 /// let mut x = Arc::new(3);
1481 /// *Arc::get_mut(&mut x).unwrap() = 4;
1482 /// assert_eq!(*x, 4);
1484 /// let _y = Arc::clone(&x);
1485 /// assert!(Arc::get_mut(&mut x).is_none());
1488 #[stable(feature = "arc_unique", since = "1.4.0")]
1489 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1490 if this.is_unique() {
1491 // This unsafety is ok because we're guaranteed that the pointer
1492 // returned is the *only* pointer that will ever be returned to T. Our
1493 // reference count is guaranteed to be 1 at this point, and we required
1494 // the Arc itself to be `mut`, so we're returning the only possible
1495 // reference to the inner data.
1496 unsafe { Some(Arc::get_mut_unchecked(this)) }
1502 /// Returns a mutable reference into the given `Arc`,
1503 /// without any check.
1505 /// See also [`get_mut`], which is safe and does appropriate checks.
1507 /// [`get_mut`]: Arc::get_mut
1511 /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
1512 /// for the duration of the returned borrow.
1513 /// This is trivially the case if no such pointers exist,
1514 /// for example immediately after `Arc::new`.
1519 /// #![feature(get_mut_unchecked)]
1521 /// use std::sync::Arc;
1523 /// let mut x = Arc::new(String::new());
1525 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1527 /// assert_eq!(*x, "foo");
1530 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1531 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1532 // We are careful to *not* create a reference covering the "count" fields, as
1533 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
1534 unsafe { &mut (*this.ptr.as_ptr()).data }
1537 /// Determine whether this is the unique reference (including weak refs) to
1538 /// the underlying data.
1540 /// Note that this requires locking the weak ref count.
1541 fn is_unique(&mut self) -> bool {
1542 // lock the weak pointer count if we appear to be the sole weak pointer
1545 // The acquire label here ensures a happens-before relationship with any
1546 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1547 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1548 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1549 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1550 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1551 // counter in `drop` -- the only access that happens when any but the last reference
1552 // is being dropped.
1553 let unique = self.inner().strong.load(Acquire) == 1;
1555 // The release write here synchronizes with a read in `downgrade`,
1556 // effectively preventing the above read of `strong` from happening
1558 self.inner().weak.store(1, Release); // release the lock
1566 #[stable(feature = "rust1", since = "1.0.0")]
1567 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1568 /// Drops the `Arc`.
1570 /// This will decrement the strong reference count. If the strong reference
1571 /// count reaches zero then the only other references (if any) are
1572 /// [`Weak`], so we `drop` the inner value.
1577 /// use std::sync::Arc;
1581 /// impl Drop for Foo {
1582 /// fn drop(&mut self) {
1583 /// println!("dropped!");
1587 /// let foo = Arc::new(Foo);
1588 /// let foo2 = Arc::clone(&foo);
1590 /// drop(foo); // Doesn't print anything
1591 /// drop(foo2); // Prints "dropped!"
1594 fn drop(&mut self) {
1595 // Because `fetch_sub` is already atomic, we do not need to synchronize
1596 // with other threads unless we are going to delete the object. This
1597 // same logic applies to the below `fetch_sub` to the `weak` count.
1598 if self.inner().strong.fetch_sub(1, Release) != 1 {
1602 // This fence is needed to prevent reordering of use of the data and
1603 // deletion of the data. Because it is marked `Release`, the decreasing
1604 // of the reference count synchronizes with this `Acquire` fence. This
1605 // means that use of the data happens before decreasing the reference
1606 // count, which happens before this fence, which happens before the
1607 // deletion of the data.
1609 // As explained in the [Boost documentation][1],
1611 // > It is important to enforce any possible access to the object in one
1612 // > thread (through an existing reference) to *happen before* deleting
1613 // > the object in a different thread. This is achieved by a "release"
1614 // > operation after dropping a reference (any access to the object
1615 // > through this reference must obviously happened before), and an
1616 // > "acquire" operation before deleting the object.
1618 // In particular, while the contents of an Arc are usually immutable, it's
1619 // possible to have interior writes to something like a Mutex<T>. Since a
1620 // Mutex is not acquired when it is deleted, we can't rely on its
1621 // synchronization logic to make writes in thread A visible to a destructor
1622 // running in thread B.
1624 // Also note that the Acquire fence here could probably be replaced with an
1625 // Acquire load, which could improve performance in highly-contended
1626 // situations. See [2].
1628 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1629 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1630 acquire!(self.inner().strong);
1638 impl Arc<dyn Any + Send + Sync> {
1640 #[stable(feature = "rc_downcast", since = "1.29.0")]
1641 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1646 /// use std::any::Any;
1647 /// use std::sync::Arc;
1649 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1650 /// if let Ok(string) = value.downcast::<String>() {
1651 /// println!("String ({}): {}", string.len(), string);
1655 /// let my_string = "Hello World".to_string();
1656 /// print_if_string(Arc::new(my_string));
1657 /// print_if_string(Arc::new(0i8));
1659 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1661 T: Any + Send + Sync + 'static,
1663 if (*self).is::<T>() {
1664 let ptr = self.ptr.cast::<ArcInner<T>>();
1666 Ok(Arc::from_inner(ptr))
1674 /// Constructs a new `Weak<T>`, without allocating any memory.
1675 /// Calling [`upgrade`] on the return value always gives [`None`].
1677 /// [`upgrade`]: Weak::upgrade
1682 /// use std::sync::Weak;
1684 /// let empty: Weak<i64> = Weak::new();
1685 /// assert!(empty.upgrade().is_none());
1687 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1689 pub fn new() -> Weak<T> {
1690 Weak { ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0") }
1694 /// Helper type to allow accessing the reference counts without
1695 /// making any assertions about the data field.
1696 struct WeakInner<'a> {
1697 weak: &'a atomic::AtomicUsize,
1698 strong: &'a atomic::AtomicUsize,
1701 impl<T: ?Sized> Weak<T> {
1702 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1704 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
1705 /// unaligned or even [`null`] otherwise.
1710 /// use std::sync::Arc;
1713 /// let strong = Arc::new("hello".to_owned());
1714 /// let weak = Arc::downgrade(&strong);
1715 /// // Both point to the same object
1716 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
1717 /// // The strong here keeps it alive, so we can still access the object.
1718 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
1721 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
1722 /// // undefined behaviour.
1723 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
1726 /// [`null`]: core::ptr::null "ptr::null"
1727 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1728 pub fn as_ptr(&self) -> *const T {
1729 let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
1731 if is_dangling(ptr) {
1732 // If the pointer is dangling, we return the sentinel directly. This cannot be
1733 // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
1736 // SAFETY: if is_dangling returns false, then the pointer is dereferencable.
1737 // The payload may be dropped at this point, and we have to maintain provenance,
1738 // so use raw pointer manipulation.
1739 unsafe { ptr::addr_of_mut!((*ptr).data) }
1743 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1745 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
1746 /// one weak reference (the weak count is not modified by this operation). It can be turned
1747 /// back into the `Weak<T>` with [`from_raw`].
1749 /// The same restrictions of accessing the target of the pointer as with
1750 /// [`as_ptr`] apply.
1755 /// use std::sync::{Arc, Weak};
1757 /// let strong = Arc::new("hello".to_owned());
1758 /// let weak = Arc::downgrade(&strong);
1759 /// let raw = weak.into_raw();
1761 /// assert_eq!(1, Arc::weak_count(&strong));
1762 /// assert_eq!("hello", unsafe { &*raw });
1764 /// drop(unsafe { Weak::from_raw(raw) });
1765 /// assert_eq!(0, Arc::weak_count(&strong));
1768 /// [`from_raw`]: Weak::from_raw
1769 /// [`as_ptr`]: Weak::as_ptr
1770 #[must_use = "`self` will be dropped if the result is not used"]
1771 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1772 pub fn into_raw(self) -> *const T {
1773 let result = self.as_ptr();
1778 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1780 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1781 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1783 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
1784 /// as these don't own anything; the method still works on them).
1788 /// The pointer must have originated from the [`into_raw`] and must still own its potential
1791 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
1792 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
1793 /// count is not modified by this operation) and therefore it must be paired with a previous
1794 /// call to [`into_raw`].
1798 /// use std::sync::{Arc, Weak};
1800 /// let strong = Arc::new("hello".to_owned());
1802 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1803 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1805 /// assert_eq!(2, Arc::weak_count(&strong));
1807 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1808 /// assert_eq!(1, Arc::weak_count(&strong));
1812 /// // Decrement the last weak count.
1813 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1816 /// [`new`]: Weak::new
1817 /// [`into_raw`]: Weak::into_raw
1818 /// [`upgrade`]: Weak::upgrade
1819 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1820 pub unsafe fn from_raw(ptr: *const T) -> Self {
1821 // See Weak::as_ptr for context on how the input pointer is derived.
1823 let ptr = if is_dangling(ptr as *mut T) {
1824 // This is a dangling Weak.
1825 ptr as *mut ArcInner<T>
1827 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
1828 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
1829 let offset = unsafe { data_offset(ptr) };
1830 // Thus, we reverse the offset to get the whole RcBox.
1831 // SAFETY: the pointer originated from a Weak, so this offset is safe.
1832 unsafe { (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) }
1835 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
1836 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
1840 impl<T: ?Sized> Weak<T> {
1841 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
1842 /// dropping of the inner value if successful.
1844 /// Returns [`None`] if the inner value has since been dropped.
1849 /// use std::sync::Arc;
1851 /// let five = Arc::new(5);
1853 /// let weak_five = Arc::downgrade(&five);
1855 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1856 /// assert!(strong_five.is_some());
1858 /// // Destroy all strong pointers.
1859 /// drop(strong_five);
1862 /// assert!(weak_five.upgrade().is_none());
1864 #[stable(feature = "arc_weak", since = "1.4.0")]
1865 pub fn upgrade(&self) -> Option<Arc<T>> {
1866 // We use a CAS loop to increment the strong count instead of a
1867 // fetch_add as this function should never take the reference count
1868 // from zero to one.
1869 let inner = self.inner()?;
1871 // Relaxed load because any write of 0 that we can observe
1872 // leaves the field in a permanently zero state (so a
1873 // "stale" read of 0 is fine), and any other value is
1874 // confirmed via the CAS below.
1875 let mut n = inner.strong.load(Relaxed);
1882 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1883 if n > MAX_REFCOUNT {
1887 // Relaxed is fine for the failure case because we don't have any expectations about the new state.
1888 // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
1889 // value can be initialized after `Weak` references have already been created. In that case, we
1890 // expect to observe the fully initialized value.
1891 match inner.strong.compare_exchange_weak(n, n + 1, Acquire, Relaxed) {
1892 Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
1893 Err(old) => n = old,
1898 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
1900 /// If `self` was created using [`Weak::new`], this will return 0.
1901 #[stable(feature = "weak_counts", since = "1.41.0")]
1902 pub fn strong_count(&self) -> usize {
1903 if let Some(inner) = self.inner() { inner.strong.load(SeqCst) } else { 0 }
1906 /// Gets an approximation of the number of `Weak` pointers pointing to this
1909 /// If `self` was created using [`Weak::new`], or if there are no remaining
1910 /// strong pointers, this will return 0.
1914 /// Due to implementation details, the returned value can be off by 1 in
1915 /// either direction when other threads are manipulating any `Arc`s or
1916 /// `Weak`s pointing to the same allocation.
1917 #[stable(feature = "weak_counts", since = "1.41.0")]
1918 pub fn weak_count(&self) -> usize {
1921 let weak = inner.weak.load(SeqCst);
1922 let strong = inner.strong.load(SeqCst);
1926 // Since we observed that there was at least one strong pointer
1927 // after reading the weak count, we know that the implicit weak
1928 // reference (present whenever any strong references are alive)
1929 // was still around when we observed the weak count, and can
1930 // therefore safely subtract it.
1937 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1938 /// (i.e., when this `Weak` was created by `Weak::new`).
1940 fn inner(&self) -> Option<WeakInner<'_>> {
1941 if is_dangling(self.ptr.as_ptr()) {
1944 // We are careful to *not* create a reference covering the "data" field, as
1945 // the field may be mutated concurrently (for example, if the last `Arc`
1946 // is dropped, the data field will be dropped in-place).
1948 let ptr = self.ptr.as_ptr();
1949 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
1954 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1955 /// [`ptr::eq`]), or if both don't point to any allocation
1956 /// (because they were created with `Weak::new()`).
1960 /// Since this compares pointers it means that `Weak::new()` will equal each
1961 /// other, even though they don't point to any allocation.
1966 /// use std::sync::Arc;
1968 /// let first_rc = Arc::new(5);
1969 /// let first = Arc::downgrade(&first_rc);
1970 /// let second = Arc::downgrade(&first_rc);
1972 /// assert!(first.ptr_eq(&second));
1974 /// let third_rc = Arc::new(5);
1975 /// let third = Arc::downgrade(&third_rc);
1977 /// assert!(!first.ptr_eq(&third));
1980 /// Comparing `Weak::new`.
1983 /// use std::sync::{Arc, Weak};
1985 /// let first = Weak::new();
1986 /// let second = Weak::new();
1987 /// assert!(first.ptr_eq(&second));
1989 /// let third_rc = Arc::new(());
1990 /// let third = Arc::downgrade(&third_rc);
1991 /// assert!(!first.ptr_eq(&third));
1994 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
1996 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1997 pub fn ptr_eq(&self, other: &Self) -> bool {
1998 self.ptr.as_ptr() == other.ptr.as_ptr()
2002 #[stable(feature = "arc_weak", since = "1.4.0")]
2003 impl<T: ?Sized> Clone for Weak<T> {
2004 /// Makes a clone of the `Weak` pointer that points to the same allocation.
2009 /// use std::sync::{Arc, Weak};
2011 /// let weak_five = Arc::downgrade(&Arc::new(5));
2013 /// let _ = Weak::clone(&weak_five);
2016 fn clone(&self) -> Weak<T> {
2017 let inner = if let Some(inner) = self.inner() {
2020 return Weak { ptr: self.ptr };
2022 // See comments in Arc::clone() for why this is relaxed. This can use a
2023 // fetch_add (ignoring the lock) because the weak count is only locked
2024 // where are *no other* weak pointers in existence. (So we can't be
2025 // running this code in that case).
2026 let old_size = inner.weak.fetch_add(1, Relaxed);
2028 // See comments in Arc::clone() for why we do this (for mem::forget).
2029 if old_size > MAX_REFCOUNT {
2033 Weak { ptr: self.ptr }
2037 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2038 impl<T> Default for Weak<T> {
2039 /// Constructs a new `Weak<T>`, without allocating memory.
2040 /// Calling [`upgrade`] on the return value always
2043 /// [`upgrade`]: Weak::upgrade
2048 /// use std::sync::Weak;
2050 /// let empty: Weak<i64> = Default::default();
2051 /// assert!(empty.upgrade().is_none());
2053 fn default() -> Weak<T> {
2058 #[stable(feature = "arc_weak", since = "1.4.0")]
2059 unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
2060 /// Drops the `Weak` pointer.
2065 /// use std::sync::{Arc, Weak};
2069 /// impl Drop for Foo {
2070 /// fn drop(&mut self) {
2071 /// println!("dropped!");
2075 /// let foo = Arc::new(Foo);
2076 /// let weak_foo = Arc::downgrade(&foo);
2077 /// let other_weak_foo = Weak::clone(&weak_foo);
2079 /// drop(weak_foo); // Doesn't print anything
2080 /// drop(foo); // Prints "dropped!"
2082 /// assert!(other_weak_foo.upgrade().is_none());
2084 fn drop(&mut self) {
2085 // If we find out that we were the last weak pointer, then its time to
2086 // deallocate the data entirely. See the discussion in Arc::drop() about
2087 // the memory orderings
2089 // It's not necessary to check for the locked state here, because the
2090 // weak count can only be locked if there was precisely one weak ref,
2091 // meaning that drop could only subsequently run ON that remaining weak
2092 // ref, which can only happen after the lock is released.
2093 let inner = if let Some(inner) = self.inner() { inner } else { return };
2095 if inner.weak.fetch_sub(1, Release) == 1 {
2096 acquire!(inner.weak);
2097 unsafe { Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr())) }
2102 #[stable(feature = "rust1", since = "1.0.0")]
2103 trait ArcEqIdent<T: ?Sized + PartialEq> {
2104 fn eq(&self, other: &Arc<T>) -> bool;
2105 fn ne(&self, other: &Arc<T>) -> bool;
2108 #[stable(feature = "rust1", since = "1.0.0")]
2109 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
2111 default fn eq(&self, other: &Arc<T>) -> bool {
2115 default fn ne(&self, other: &Arc<T>) -> bool {
2120 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
2121 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
2122 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
2123 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
2124 /// the same value, than two `&T`s.
2126 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
2127 #[stable(feature = "rust1", since = "1.0.0")]
2128 impl<T: ?Sized + crate::rc::MarkerEq> ArcEqIdent<T> for Arc<T> {
2130 fn eq(&self, other: &Arc<T>) -> bool {
2131 Arc::ptr_eq(self, other) || **self == **other
2135 fn ne(&self, other: &Arc<T>) -> bool {
2136 !Arc::ptr_eq(self, other) && **self != **other
2140 #[stable(feature = "rust1", since = "1.0.0")]
2141 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
2142 /// Equality for two `Arc`s.
2144 /// Two `Arc`s are equal if their inner values are equal, even if they are
2145 /// stored in different allocation.
2147 /// If `T` also implements `Eq` (implying reflexivity of equality),
2148 /// two `Arc`s that point to the same allocation are always equal.
2153 /// use std::sync::Arc;
2155 /// let five = Arc::new(5);
2157 /// assert!(five == Arc::new(5));
2160 fn eq(&self, other: &Arc<T>) -> bool {
2161 ArcEqIdent::eq(self, other)
2164 /// Inequality for two `Arc`s.
2166 /// Two `Arc`s are unequal if their inner values are unequal.
2168 /// If `T` also implements `Eq` (implying reflexivity of equality),
2169 /// two `Arc`s that point to the same value are never unequal.
2174 /// use std::sync::Arc;
2176 /// let five = Arc::new(5);
2178 /// assert!(five != Arc::new(6));
2181 fn ne(&self, other: &Arc<T>) -> bool {
2182 ArcEqIdent::ne(self, other)
2186 #[stable(feature = "rust1", since = "1.0.0")]
2187 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
2188 /// Partial comparison for two `Arc`s.
2190 /// The two are compared by calling `partial_cmp()` on their inner values.
2195 /// use std::sync::Arc;
2196 /// use std::cmp::Ordering;
2198 /// let five = Arc::new(5);
2200 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
2202 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
2203 (**self).partial_cmp(&**other)
2206 /// Less-than comparison for two `Arc`s.
2208 /// The two are compared by calling `<` on their inner values.
2213 /// use std::sync::Arc;
2215 /// let five = Arc::new(5);
2217 /// assert!(five < Arc::new(6));
2219 fn lt(&self, other: &Arc<T>) -> bool {
2220 *(*self) < *(*other)
2223 /// 'Less than or equal to' comparison for two `Arc`s.
2225 /// The two are compared by calling `<=` on their inner values.
2230 /// use std::sync::Arc;
2232 /// let five = Arc::new(5);
2234 /// assert!(five <= Arc::new(5));
2236 fn le(&self, other: &Arc<T>) -> bool {
2237 *(*self) <= *(*other)
2240 /// Greater-than comparison for two `Arc`s.
2242 /// The two are compared by calling `>` on their inner values.
2247 /// use std::sync::Arc;
2249 /// let five = Arc::new(5);
2251 /// assert!(five > Arc::new(4));
2253 fn gt(&self, other: &Arc<T>) -> bool {
2254 *(*self) > *(*other)
2257 /// 'Greater than or equal to' comparison for two `Arc`s.
2259 /// The two are compared by calling `>=` on their inner values.
2264 /// use std::sync::Arc;
2266 /// let five = Arc::new(5);
2268 /// assert!(five >= Arc::new(5));
2270 fn ge(&self, other: &Arc<T>) -> bool {
2271 *(*self) >= *(*other)
2274 #[stable(feature = "rust1", since = "1.0.0")]
2275 impl<T: ?Sized + Ord> Ord for Arc<T> {
2276 /// Comparison for two `Arc`s.
2278 /// The two are compared by calling `cmp()` on their inner values.
2283 /// use std::sync::Arc;
2284 /// use std::cmp::Ordering;
2286 /// let five = Arc::new(5);
2288 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
2290 fn cmp(&self, other: &Arc<T>) -> Ordering {
2291 (**self).cmp(&**other)
2294 #[stable(feature = "rust1", since = "1.0.0")]
2295 impl<T: ?Sized + Eq> Eq for Arc<T> {}
2297 #[stable(feature = "rust1", since = "1.0.0")]
2298 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
2299 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2300 fmt::Display::fmt(&**self, f)
2304 #[stable(feature = "rust1", since = "1.0.0")]
2305 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
2306 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2307 fmt::Debug::fmt(&**self, f)
2311 #[stable(feature = "rust1", since = "1.0.0")]
2312 impl<T: ?Sized> fmt::Pointer for Arc<T> {
2313 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2314 fmt::Pointer::fmt(&(&**self as *const T), f)
2318 #[cfg(not(no_global_oom_handling))]
2319 #[stable(feature = "rust1", since = "1.0.0")]
2320 impl<T: Default> Default for Arc<T> {
2321 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
2326 /// use std::sync::Arc;
2328 /// let x: Arc<i32> = Default::default();
2329 /// assert_eq!(*x, 0);
2331 fn default() -> Arc<T> {
2332 Arc::new(Default::default())
2336 #[stable(feature = "rust1", since = "1.0.0")]
2337 impl<T: ?Sized + Hash> Hash for Arc<T> {
2338 fn hash<H: Hasher>(&self, state: &mut H) {
2339 (**self).hash(state)
2343 #[cfg(not(no_global_oom_handling))]
2344 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
2345 impl<T> From<T> for Arc<T> {
2346 /// Converts a `T` into an `Arc<T>`
2348 /// The conversion moves the value into a
2349 /// newly allocated `Arc`. It is equivalent to
2350 /// calling `Arc::new(t)`.
2354 /// # use std::sync::Arc;
2356 /// let arc = Arc::new(5);
2358 /// assert_eq!(Arc::from(x), arc);
2360 fn from(t: T) -> Self {
2365 #[cfg(not(no_global_oom_handling))]
2366 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2367 impl<T: Clone> From<&[T]> for Arc<[T]> {
2368 /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
2373 /// # use std::sync::Arc;
2374 /// let original: &[i32] = &[1, 2, 3];
2375 /// let shared: Arc<[i32]> = Arc::from(original);
2376 /// assert_eq!(&[1, 2, 3], &shared[..]);
2379 fn from(v: &[T]) -> Arc<[T]> {
2380 <Self as ArcFromSlice<T>>::from_slice(v)
2384 #[cfg(not(no_global_oom_handling))]
2385 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2386 impl From<&str> for Arc<str> {
2387 /// Allocate a reference-counted `str` and copy `v` into it.
2392 /// # use std::sync::Arc;
2393 /// let shared: Arc<str> = Arc::from("eggplant");
2394 /// assert_eq!("eggplant", &shared[..]);
2397 fn from(v: &str) -> Arc<str> {
2398 let arc = Arc::<[u8]>::from(v.as_bytes());
2399 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
2403 #[cfg(not(no_global_oom_handling))]
2404 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2405 impl From<String> for Arc<str> {
2406 /// Allocate a reference-counted `str` and copy `v` into it.
2411 /// # use std::sync::Arc;
2412 /// let unique: String = "eggplant".to_owned();
2413 /// let shared: Arc<str> = Arc::from(unique);
2414 /// assert_eq!("eggplant", &shared[..]);
2417 fn from(v: String) -> Arc<str> {
2422 #[cfg(not(no_global_oom_handling))]
2423 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2424 impl<T: ?Sized> From<Box<T>> for Arc<T> {
2425 /// Move a boxed object to a new, reference-counted allocation.
2430 /// # use std::sync::Arc;
2431 /// let unique: Box<str> = Box::from("eggplant");
2432 /// let shared: Arc<str> = Arc::from(unique);
2433 /// assert_eq!("eggplant", &shared[..]);
2436 fn from(v: Box<T>) -> Arc<T> {
2441 #[cfg(not(no_global_oom_handling))]
2442 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2443 impl<T> From<Vec<T>> for Arc<[T]> {
2444 /// Allocate a reference-counted slice and move `v`'s items into it.
2449 /// # use std::sync::Arc;
2450 /// let unique: Vec<i32> = vec![1, 2, 3];
2451 /// let shared: Arc<[i32]> = Arc::from(unique);
2452 /// assert_eq!(&[1, 2, 3], &shared[..]);
2455 fn from(mut v: Vec<T>) -> Arc<[T]> {
2457 let arc = Arc::copy_from_slice(&v);
2459 // Allow the Vec to free its memory, but not destroy its contents
2467 #[stable(feature = "shared_from_cow", since = "1.45.0")]
2468 impl<'a, B> From<Cow<'a, B>> for Arc<B>
2470 B: ToOwned + ?Sized,
2471 Arc<B>: From<&'a B> + From<B::Owned>,
2473 /// Create an atomically reference-counted pointer from
2474 /// a clone-on-write pointer by copying its content.
2479 /// # use std::sync::Arc;
2480 /// # use std::borrow::Cow;
2481 /// let cow: Cow<str> = Cow::Borrowed("eggplant");
2482 /// let shared: Arc<str> = Arc::from(cow);
2483 /// assert_eq!("eggplant", &shared[..]);
2486 fn from(cow: Cow<'a, B>) -> Arc<B> {
2488 Cow::Borrowed(s) => Arc::from(s),
2489 Cow::Owned(s) => Arc::from(s),
2494 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
2495 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]> {
2496 type Error = Arc<[T]>;
2498 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2499 if boxed_slice.len() == N {
2500 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2507 #[cfg(not(no_global_oom_handling))]
2508 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2509 impl<T> iter::FromIterator<T> for Arc<[T]> {
2510 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2512 /// # Performance characteristics
2514 /// ## The general case
2516 /// In the general case, collecting into `Arc<[T]>` is done by first
2517 /// collecting into a `Vec<T>`. That is, when writing the following:
2520 /// # use std::sync::Arc;
2521 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2522 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2525 /// this behaves as if we wrote:
2528 /// # use std::sync::Arc;
2529 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2530 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2531 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2532 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2535 /// This will allocate as many times as needed for constructing the `Vec<T>`
2536 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2538 /// ## Iterators of known length
2540 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2541 /// a single allocation will be made for the `Arc<[T]>`. For example:
2544 /// # use std::sync::Arc;
2545 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2546 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2548 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2549 ToArcSlice::to_arc_slice(iter.into_iter())
2553 /// Specialization trait used for collecting into `Arc<[T]>`.
2554 trait ToArcSlice<T>: Iterator<Item = T> + Sized {
2555 fn to_arc_slice(self) -> Arc<[T]>;
2558 #[cfg(not(no_global_oom_handling))]
2559 impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
2560 default fn to_arc_slice(self) -> Arc<[T]> {
2561 self.collect::<Vec<T>>().into()
2565 #[cfg(not(no_global_oom_handling))]
2566 impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
2567 fn to_arc_slice(self) -> Arc<[T]> {
2568 // This is the case for a `TrustedLen` iterator.
2569 let (low, high) = self.size_hint();
2570 if let Some(high) = high {
2574 "TrustedLen iterator's size hint is not exact: {:?}",
2579 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2580 Arc::from_iter_exact(self, low)
2583 // TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
2584 // length exceeding `usize::MAX`.
2585 // The default implementation would collect into a vec which would panic.
2586 // Thus we panic here immediately without invoking `Vec` code.
2587 panic!("capacity overflow");
2592 #[stable(feature = "rust1", since = "1.0.0")]
2593 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2594 fn borrow(&self) -> &T {
2599 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2600 impl<T: ?Sized> AsRef<T> for Arc<T> {
2601 fn as_ref(&self) -> &T {
2606 #[stable(feature = "pin", since = "1.33.0")]
2607 impl<T: ?Sized> Unpin for Arc<T> {}
2609 /// Get the offset within an `ArcInner` for the payload behind a pointer.
2613 /// The pointer must point to (and have valid metadata for) a previously
2614 /// valid instance of T, but the T is allowed to be dropped.
2615 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2616 // Align the unsized value to the end of the ArcInner.
2617 // Because RcBox is repr(C), it will always be the last field in memory.
2618 // SAFETY: since the only unsized types possible are slices, trait objects,
2619 // and extern types, the input safety requirement is currently enough to
2620 // satisfy the requirements of align_of_val_raw; this is an implementation
2621 // detail of the language that must not be relied upon outside of std.
2622 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2626 fn data_offset_align(align: usize) -> isize {
2627 let layout = Layout::new::<ArcInner<()>>();
2628 (layout.size() + layout.padding_needed_for(align)) as isize